For those using trainers, there are many reports of people being unable to reach their true FTP indoors, although the reasons are unclear. Possibly something to do with overheating or simply psychology, but for me when I had a power tap I could never get close to my true FTP indoors even when enduring twice the discomfort I would have on road.

But not everyone experiences this effect, so who knows.

I can certainly top my maximum HR outdoors while racing by 10 bpm when compared to an indoors trainer. So output must be higher overall IRL

I really struggle with efforts above threshold on trainer – I’ve recently been doing 60 seconds on/off x6 and typically averaging around 350w outdoors, top end of Z5

Same workout on the Kickr and I’m shipping around 25w, towards the bottom end of Z5

Interestingly enough FTP tests outdoors and indoors correlate quite well!

It’s well known that many folks require an indoor and outdoor ftp number, as the 2 don’t often align. Reasons given are overheating, motivation, and how the force through the pedalling stroke is different on a trainer due to inertia. The latter of these is lessened the better the turbo you have.

From efforts indoors and out I estimate my indoor ftp to be about 10% lower than outdoors, using power reading from same stages device.

The other point is that depending on the indoor turbo, the power readings may be quite a bit out. I have a flux and I can see it shows considerably different (lower) power outputs than the stages I have on the bike I use on it. Other trainers I’ve had read considerably over the stages however.

So..If you only have a trainer and no power meter the ftp you think you have may be out by a significant margin!

I think I am correct in saying the 20min effort counts as an FTP test?

Sort of not. People keep quoting 20-minute numbers taken from rides, races etc, but that’s not quite how ftp works, that’s just a 20-minute effort time.

First, with dedicated ftp testing, they’re usually preceded by some short, hard efforts so your anaerobic system doesn’t prop up your aerobic output, which is what ftp aims to measure.

Second, the theory of ftp is that it’s your average power output over 60 minutes. Actually riding for 60 minute is super dull and really hard, so ftp measured over 20 minutes, but at 95% of your 20 minute power output.

I’m not sure how Zwift calculates ftp from rides because, if you think about it, a 20-minute ride segment could include a bunch of over threshold efforts interspersed with stuff just below threshold. If you were someone with disproportionately high MAP or five-minute power and good recovery, I suppose, in theory, you could actually inflate your ftp as a result. But that’s not much help to you as what you’re trying to measure is that steady state for an hour figure.

As far as the outdoor v indoor figures go. It’s always seemed to me that it doesn’t really matter. You just use the one that relates to your normal training: if you ride indoors use your indoor ftp, if outdoors, use the outdoor one.

Finally ftp is all very well, but if you think about it, it’s just one aspect of your fitness. Someone who can make repeated hard efforts above threshold and recover and repeat could have the same ftp as you on paper, but in the theoretical real world can kick your butt by simply dragging you repeatedly into the red until you can’t recover.

Anyway…

Of course, FTP matters for doing FTP training. Otherwise the only thing that matters is your lap time!

Finally ftp is all very well, but if you think about it, it’s just one aspect of your fitness. Someone who can make repeated hard efforts above threshold and recover and repeat could have the same ftp as you on paper, but in the theoretical real world can kick your butt by simply dragging you repeatedly into the red until you can’t recover.

This is 100% spot on. Unless you are putting in long alpine efforts on climbs, or riding a tt, then your ftp is a good gauge of fitness but nothing more. I found this out the hard way last ‘mtb race’ I did. I’m fine at sitting at threshold for an hr, but within 5 min I was in the red and cooked for the rest of the lap.

Having a power meter is useful for knowing at what power you combust however, so on the road it makes judging efforts far easier. I don’t have one for the mtb, but even if I did I think the nature of most mtb rides means power, or at least ftp, is irrelevant.

I’m in on this.

I’d like to get back to 4w/kg once my knee is sorted. Hopefully riding by the end of September & I’d like to hit 4 by Spring next year. The last time I managed it I was about 70kg & 280w. Before my injury I was at 250w & 71.5kg..